Centrifugal force is useful for separation of solids from streams of fluids. Such separation can find application in various industrial processes, for example in removing of catalyst particles in fluidized bed catalytic cracking operations, and separation of ash from flue gases.
Fluids, when rotating around a central axis, will be accelerating toward the central axis, and inertia of the solids will force the solids outward away from the central axis. Outward flow of particles will be against the flow of fluids. Fluids containing fewer solids are then typically withdrawn from the center axis of rotation, and the solids are removed from the radially outer surface of the separator. The rate of flow of solids outward is limited by the resistance of the fluid. This rate is dictated by Stokes Law. Practically, only about five micron sized particles can be separated by conventional cyclone separators.
U.S. Pat. No. 5,073,177 suggests an apparatus for cyclonic separation of solids from a gas stream. This apparatus includes a rotating element of a large number of small conduits so that the particles in each small conduit only travel a short distance before contacting a surface, on which the particles will agglomerate. The element is said to rotate at 3000 rpm, to maximize the centrifugal force acting upon the particles being separated. The speed of rotation of the particles is limited to the speed at which the element can be rotated. This high rotational speed results in a relatively high maintenance apparatus. It is desirable to have a greater rotational speed imparted to the fluids without a rotating element that rotates at a high speed.
Dutch patent application No. 8901841 discloses a method of removing a selected gaseous component from a stream of fluid containing a plurality of gaseous components, wherein the stream is induced to flow at a supersonic velocity through a conduit so as to decrease the temperature of the fluid in the conduit to below a temperature at which the selected component condenses. The conduit is provided with swirl imparting means to impart a swirling motion to the stream of fluid flowing at supersonic velocity. The condensed particles are extracted in a first outlet stream from a radially outer section of the stream and the remaining fluid is collected in a second outlet stream from a central part of the stream. The velocity in the radially outer section and in the central part of the stream is supersonic.
In an embodiment of the device for separating a gas from a gas mixture as disclosed in NL-8901841, separate shock waves occur in the first and second outlet streams, leading to a relatively large flow resistance of the fluid. Furthermore, the separation efficiency is relatively low so that substantial amounts of the condensed particles are still present in the second outlet stream. This reference does not suggest utilizing such an apparatus for separation of solids from fluids.